Abstract
AbstractConceptual and methodological tools from behavioral ecology can inform studies of habitat quality, and their potential for evaluating habitat restoration in conservation efforts is explored here. Such approaches provide mechanistic detail in understanding the relationship between organisms and their habitats and are thus more informative than correlations between density and habitat characteristics. Several Pacific salmon species have been the target of habitat restoration efforts for the past 2–3 decades, but most post‐restoration effectiveness studies have been limited to correlative data described above. In mark–recapture assays from four different study years, the affinity of sub‐yearling Chinook salmon (Oncorhynchus tschawytscha) and steelhead (O. mykiss) for stream pools restored with or created by engineered log structures was greater than that for pools without restoration, though with high interannual variability. From corresponding distribution and density data, it was clear that habitat affinity data are not always concordant with single observations of density. The same was true of the correlation between either affinity or density and physical characteristics of pools, although depth and current velocity had some explanatory power for both responses in Chinook. Movement into pools by Chinook during the assays indicated that restored pools can support more immigrants at a given density than can unrestored pools; however, no such pattern emerged for steelhead. Variation among individuals in body condition has implications for population‐wide fitness, and such low variation was correlated with stronger affinity for pools in Chinook regardless of restoration status. This suggests that pools may mediate habitat‐related trade‐offs and that restoring them might have a positive effect on fitness. Thus affinity, immigration, and condition data give much‐needed mechanistic indication of habitat selection for restored habitat via an apparent capacity increase and those potential fitness benefits. This is stronger support for restoration effectiveness than density differences alone because density data (1) may simply indicate redistribution of fish from poor to good habitats and (2) are not adequate to show correlations between restoration and positive change in traits correlated with fitness.
Highlights
For stream pools restored with or created by engineered log structures was greater than that for pools without restoration, though with high interannual variability
Structures placed in streams to create or augment pools are a significant part of restoration efforts in the Pacific Northwest (Roni et al 2002) because these habitats are important to the rearing phase of the life cycle of salmonids (Roni et al 2008)
The inferential power of the results is limited by inadequate replication of structures and/or observational scale mismatched with treatment scale (Roni et al 2002, McMillan et al 2013, Freedman et al 2016, Polivka et al 2019), limiting the ability to detect and quantify the seasonal, annual, and among-species distribution patterns (Bradford and Higgins 2001). This does not necessarily indicate poorly placed or targeted restoration activities, but rather the need for consideration of more robust metrics, including those that more directly describe habitat selection (Conrad et al 2011, Kotler et al 2016). These metrics include: 1) site fidelity, 2) density dependent movement, and 3) dependence of traits correlated with fitness, like body condition, on habitat selection related movement, all of which can be studied with relatively simple mark-recapture assays
Summary
The Entiat River is a major tributary sub-basin of the Interior Columbia River Basin in north central Washington State, USA. There, a common habitat restoration action is construction of in-stream structures to create rearing pools for sub-yearling Chinook salmon (listed as endangered) and steelhead (listed as threatened). Predation risk primarily comes from birds (belted kingfisher, Ceryle alcyon; great blue heron, Ardea herodias) and semi-aquatic mammals Larger, predatory fish such as resident and fluvial bull trout (Salvelinus confluentus) have been observed in deeper pools created by larger in-stream structures such as channel-spanning weirs, but not in the smaller pools created by the engineered log jams (ELJs) that comprised restoration projects in the river. The reach association of the restored and unrestored pools is a convention for identification rather than an experimental effect
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